When a Forward to a friend form is filled out, are the friends e-mail addresses added to a subscriber list automatically?
No, when a person completes the Forward to a Friend form and sends information to other email addresses, those addresses are not added to any subscriber lists in the Emal Marketing module.
No future email messages will be sent to any email address collected via a Forward to a Friend form. However, you can view these email addresses in the form submission information in the Forms module. You can manually add them to a subscriber list.
Note that you should use caution before adding this type of collected email address to a subscriber list as the email recipient has not "opted in" to receive emails from the site. Sending messages to recipients that have not opted to receive emails means that you may be spamming these addresses and future emails run the risk of getting blocked or blacklisted for this.